The present invention relates in general to a patient respiratory ventilator and, more particularly, to a system and a method for circuit compliance compensated volume assured pressure control in a patient respiratory ventilator.
In order to accurately deliver at least a set assured volume to the patient during a pressure-control mode of ventilation, a ventilator must compensate for patient circuit compliance. This is particularly crucial for neonatal patients for whom the circuit compliance is often much larger than the lung compliance. Without adequate compensation of the patient circuit compliance, inaccurate volume and flow may be delivered to the patient. Included in the prior art are several approaches of patient circuit compliance. These prior art approaches have been designed and applied to currently available ventilators such as the AVEA® Comprehensive Ventilator, commercially available from Viasys Healthcare Inc., assignee of the subject disclosure.
Unfortunately, most of the approaches, while compensating for the patient circuit compliance, inevitably cause gas trapping and auto PEEP, which consequently impacts the ability of the patient to exhale the delivered tidal volume. Therefore, many ventilators of the prior art do not allow application of patient circuit compliance to neonatal patients due to the stringent volume precision requirement. The burden of providing accurate volume delivery is thus shifted to the clinician.
Currently, known volume assurance algorithms, such as may be used with the AVEA® Comprehensive Ventilator mentioned above as well as other prior art ventilators, are typically only suitable for pediatric and adult-sized patients. In such volume assurance algorithms, when a volume assurance is set in a pressure-controlled mode, the inspiratory flow controller command is the maximum of the pressure-control flow command and a decelerating flow command. Therefore, depending on the set inspiratory pressure, assured volume, airway resistance, lung compliance and circuit compliance, the breath delivery can result in a pressure-controlled breath, a volume controlled breath, or a hybrid of pressure and volume controlled breath.
By setting the volume assurance, a decelerating flow command profile for the current breath is generated by using an estimate of circuit volume from the previous breath, the set assured volume and the set inspiratory time. During inspiration, the decelerating flow command will be terminated if the system delivered volume, as measured by the inspiratory sensor, exceeds the set assured volume and the circuit volume that is computed during the breath. The breath is cycled to exhalation control when the set inspiratory time is reached and the system delivered volume exceeds the set assured volume & circuit volume computed during the breath.
Using a MATLAB®-based rapid prototyping HITL system, simulated applications of the above-mentioned algorithm to different patient sizes, including neonates, have been performed and certain deficiencies in the algorithm have been discovered. Firstly, during ventilation of a patient, the system may not achieve the desired volume delivery within the set inspiratory time. This is particularly a problem for neonate patients where the circuit to lung compliance ratio can be large. If the delivered system volume fails to reach the set assured and circuit volumes within the set inspiratory time, the inspiratory time is extended to allow time for volume delivery until the I:E ratio limit is eventually reached. In most cases, the system consecutively reaches the I:E ratio limit for up to 5 breaths before stabilization. Secondly, for cases where the circuit to lung compliance ratio is as high as 13:1 and a minimal set inspiratory time is set, volume delivery errors may occur if the decelerating flow command reaches the allowable maximum flow command.
This can be a problem when the airway resistance is high and additional flow is required to compensate for the circuit compliance. The I:E limit will be reached in these cases because the flow required to compensate for the circuit volume cannot be achieved. Thirdly, because net system delivery volume is not used, exhalation valve leaks during inspiration are not accounted for during volume delivery. This can significantly affect the accuracy of volume delivery. Fourthly, excess volume delivery due to flow control valve closing dynamics is not accounted for during volume delivery. Fortunately, this may not be as critical in the pressure-control mode with a set volume assurance since only a minimum volume is required.